Urea-prill anti-caking process

ABSTRACT

Hot urea prills can be made to resist caking by cooling them quickly under quiescent conditions. To accomplish this, the hot prills are collected in a cooling chamber and a flow of air at a temperature not exceeding about 130* F is directed through the prills so that the prill temperature is quickly reduced. Throughout the cooling step the prills are maintained in a quiescent state so that a glassy surface is maintained on each prill. Such surface helps reduce subsequent caking.

United States Patent Griesheimer et al.

[ Aug. 22, 1972 22 Filed:

I [54] UREA-PRILL ANTI-CAKING PROCESS [72] Inventors: George T.Griesheimer, 496i Normandy Ave., Memphis, Tenn. 38117; Joe I. Chance,4142 Fanon Ave., Memphis, Tenn. 38122 Feb. 9, 1970 [21] Appl. No.2 9,835

52 us. c1 ..264/14, 264/7 [51] Int. Cl. ..B01j 2/04 [58] Field of Search..264/14, 7', 34/13, 20, 62

[56] References Cited UNITED STATES PATENTS 3,049,420 8/1962 Weiland..264/14 3,130,225 4/1964 Friend ..264/14 2,887,724 5/1959 Bettes, Jr..264/14 FOREIGN PATENTS OR APPLICATIONS 852,650 9/1970 Canada ..264/14Primary Examiner-Robert F. White Assistant Examiner-J. R. HallAttorney-Charles L. Harness and Kenneth E. Prince [57] ABSTRACT Hot ureaprills can be made to resist caking by cooling them quickly underquiescent conditions. To accomplish this, the hot prills are collectedin a cooling chamber and a flow of air at a temperature not exceedingabout 130 F is directed through the prills so that the prill temperatureis quickly reduced. Throughout the cooling step the prills aremaintained in a quiescent state so that a glassy surface is maintainedon each prill. Such surface helps reduce subsequent caking.

9 Claim, 7 Drawing Figures PATENTEDAUBZZ I972 SHEEI 1 0F 4 George 7.Gr/es/r e/mer Joe I. Chance INVENTORS BY w... L. H-

AT'TURNI'IY PATENTEDwszz I972 SHEET 2 0F 4 mm N mat NM,

Joe Chance INVENTORS ATTORNEY;

PATENTEUnuczz I972 AIR FLOW, THOUSAND STANDARO CUB/C F? MINUTE SHEET 3UF 4 50 I 45 AIRFLOW Vs AIR Tam ms:

m ms UREA PRILL COOLER or Fla 3 ans/s: 4oq TN% UREA PRILL Pn/LLs m0I60'F 35 PRILLS our-0 mo r NET HEAT nsuovzo 781,800 any/m. 3o

so so 00 AIR mm 0155, "F

George .7. anbshe/mer Joe 1 Chance @wa a L. Mam y INVENTORS PATENIEDwszzI972 SHEET 6 0F 4 b bbx INVENTORS George 7. Gneshe/mer SIVOUJIN W6777/Ud I Joe Chance ATTORNEY UREA-PRILL ANTI-CAKING PROCESS Thisinvention relates to the prevention of caking of urea prills.

Urea prills taken directly from the prill tower and bagged tend to cakebadly on storage, and this tendency to caking is greatly exaggerated bythe modern warehouse practice of stacking bags of prilled urea six oreight tiers high on pallets.

Even in short-term storage, bags of conventionally prepared urea prillsfrequently will be found with more than 90 percent of the urea solidlycaked. Caking is an expensive nuisance, both for the farmer and for thechemical user. It is particularly annoying in connection with farmfertilizer spreaders, for few farmers have any equipment for breaking upsolid chunks of urea. The chemical user finds it necessary to regrindbadly caked urea, and must put up with dusting problems which frequentlyhe is ill equipped to handle.

Normally, the prills, after falling to the bottom of the prilling tower,are elevated to a rotary cooler and inclined tubular device throughwhich a current of air is forced as the prills roll down the incline tothe cooler exit. To assure cooling, the tube is rotated, spillingquantities of prills through the air flow after the manner of a concretemixer. 1

An examination of the product of this process will show that the prillsare significantly abraded, for very fine discrete particles will beobserved sticking to the surface of each prill, and the hard glassysurface which otherwise envelops the prill as it falls to the bottom ofthe prilling tower has been injured.

A reasonable explanation of the tendency of prills to cake is that thevery fine powdered material which surrounds the prill absorbsatmospheric water or water vapor. The moisture subsequently dries out toindefinite degrees, causing the dissolved and re-dried urea to form acement between particle and particle, which locks the whole into arigid, stubborn cake.

We have discovered that if prilled urea, immediately upon its exit fromthe prilling tower, is run to a device where the prills may rest in asubstantially quiescent state until they have reached a low temperatureat least as low as 130 F. and preferably even room temperature, when theprills are taken from such a cooling device, their surfaces remainglassy and unabraded, and are relatively free of urea dust. Such prillsresist caking much better than-do the prills which are passed through arotary drum type cooler. In fact, conventional liquid anticaking agents(as hereinafter discussed) can be added to these quick-cooled prills inconventional rotary mixing equipment, and negligible abrasion willresult. Preferably, of course, the liquid additives are added to theprills while minimizing prill-toprill contact.

We have found it preferable, and have determined that very effectivenon-caking conditions can be established if, in addition to thequick-cooling provided by this invention, the prills, as they fall into,through, or out of the cooler, are coated with a thin, possibly only amono-molecular, film of a liquid which is strongly adsorbed on the ureaprill surface. The precise action of the treating fluid in greatlyreducing the tendency of the prills to cake is not completelyunderstood, for essentially hydrophobic liquids, slightly solubleliquids, and liquids which are infinitely soluble in water, have beenfound to be effective anticaking sprays for our quick-cooled prills.

After the prills have been cooled by the process of this invention andthen optionally sprayed or coated with a liquid as described, the prillsmay be bagged, handled in normal manners both in freight transport andin warehousing, and, particularly in warehousing, can be compacted withconsiderable weight. When bags of urea treated in this manner areopened, instead of the hard lump or lumps which otherwise would befound, a very substantial amount of the contents of the bag will befree-flowing prills. The lumps which do exist are not the hard andstubborn concretions which previously have been experienced, but ratherare aggregations of loosely adherent particles which break up withlittle mechanical effort.

It is the object of the present invention to eliminate, so far aspossible, caking and maintain the free-flowing characteristics whichoriginally were possessed by the prills at the moment of theirmanufacture. The process by which we achieve superior results isillustrated in the accompanying drawings wherein:

FIG. LA is a diagrammatic representation of a cooling method, showingthe batch (or static) cooler in the prill-receiving position.

FIG. l-B shows the same cooler with cover and air tubes attached.

FIG. l-C shows the batch cooler discharging the cooled urea prills.

FIG. 2 is an elevated view of the static cooler with part of itsexterior cut away to show a portion of the retaining screen.

FIG. 3 is a flow diagram of a continuous or semistatic urea prill coolersystem.

FIG. 4 is a graph showing air flow versus air temperature rise in amoving urea prill bed such as that of FIG. 3.

FIG. 5 is a graph showing terminal velocity of urea prills.

Turning now to FIG. l-A, the static cooler 10 receives prills which havebeen lifted from the bottom of the prilling tower 11 to said cooler byan elevator belt 12. The prills fall into the cooler and accumulate on asupport screen 13 until their level reaches the top of the sight glass(shown at 23, FIG. 2) or other suitable level. The elevator belt is thenshut off or the flow directed to an empty cooler (not shown).

Turning to FIG. l-B, air tubes 14 and 15 are attached to air ducts 16and 17 respectively. Air, suitably at 60-85 F. is forced into the coolerthrough 14 and exits through 15.

Air cooling is continued until the temperature of the prills falls atleast down or to 130 F., and preferably to a temperature in the range of-l 10 F. When the prills reach this lower temperature, (requiring 10minutes to 1 hour), air flow through the side arm 14 is cut off, thetubes are disconnected, and the static cooler is up-ended by beingrotated on its trunions, 22 through half a revolution.

When the cooler is upside down, as in FIG. l-C, the prills flow out ofthe pipe 17, optionally on to a sizing screen (not shown) and then passalong any desired form of transport apparatus 18, to a conventionalbagging device (not shown).

In FIG. 2, the batch cooler is shown generally at 10. It is equippedwith a separable conically shaped cover 19 which has duct 17 for theattachment of an air tube. (Such attachment is shown in FIG. L3.) Thecover is removed when the cooler is receiving prills and is replacedwhen cooling air is passed through the cooler. The cover 19 isconveniently fastened to the cooler by means of nuts and bolts (notshown). After the cooler is filled with prills to the desired level, thecover is fastened on and cooling air is passed through the cooler. Afterthe cooling step, the cooled prills are discharged. For this step, theair tubes are disconnected and the cooler is inverted, turning ontrunions 22 which rest in cooler support 21.

The above described batch static cooler outlines the invention in itssimplest aspect. In large scale commercial practice, essentially thesame concept is applied on a continuous basis, with prills continuouslyentering and leaving the cooler, meanwhile presenting a bed of prills toa cooling air stream. The air stream is recooled and recycled. Thisembodiment is set forth diagrammatically in FIG. 3 and is describedbelow.

The main piece of equipment used in operating this inventioncontinuously is the urea cooler shown generally at 30. This cooler isabout 8 feet in diameter by 8 feet long (straight side). The cooler iscovered by a dome, 31 which contains an entrance port for the urea prillfeeder 32. The urea prill feeder is a tube equipped at its exit end 33(within the cooler 30) with a hinged gate (not shown) which is pushedopen by the fall of prills down the tube, but which closes shut by itsown weight at other times to prevent the escape of vapors back up thefeeding tube 33 during the operation. The cooler 30 is equipped with amotor-driven paddle 34, which is a sensing device. This paddle turns ata rate of about 5 rpm, and during the feeding operation, when the levelof urea prills rises to meet the paddle, the torque on the paddleincreases, thereby actuating electrically the motor-driven star valve 35at the bottom of the static cooler, to increase its delivery of cooledprills.

In one embodiment of this invention the star valve 35 controls theentrance of the urea prills to the cooler 30 in the following manner:when the drag on the paddle 34 is zero, the star valve will not move;when the paddle drag is normal, the star valve will move at a normalrate; and, when the paddle drag is maximum, the star valve will move ata rate in excess of the capacity of the feed belt supplying urea prills.Other variations, include an on-ofl operation with the star valvecapacity larger than the belt feed rate or a high level and low levelsensing element to either stop the feed belt or star valve as required.

Preferably, the depth of prills in the cooler 30 is kept at about 3.5feet. Under the preferred conditions, the air velocity through this bedwill be about 2.74 feet per second, which permits the fall into the bedof all prills except those which have a diameter below 140 microns.Since prills smaller than 140 microns occur only in very small amountsin normal prilling operations (about 1 part per 1,000), such undersizedmaterial is simply swept around in air recycled to the air cooler 36. Abed of this size for the apparatus stated will provide a prill volume ofabout l73 cubic feet. Air at about 17.0 psia from an air cooler 36 at 70F. and at percent relative humidity (i.e., 37,900 lb/hr dry air admixedwith 606.4 lb/hr water vapor) is transported through a pipe 37 to theprill cooler 30 via an air inlet port 38, and an air sparger 39, whichpreferably discharges downward to avoid plugging its air holes with fineurea particles. The cool air passes through the prill bed and isrecovered at substantially atmospheric pressure on the top side of thebed, with a lowering in pressure of 3.17 psi. Under equilibriumconditions, average residence time of the prills in the cooler is about15 minutes. When the operation reaches equilibrium, the prills leave thecooler 30 at about 100 F. The cooling air leaves the cooler at about F.and with a relative humidity of 6.6 percent, through the air exit port40 at 8,260 standard cubic feet/minute and is transported to an airblower 41 at atmospheric pressure. The air blower has a capacity of 382brake horsepower. It raises the temperature to 215 F. and the pressureto 21.07psia. The air blower employed in this invention has the effectof compressing the air, thus causing the air to exhibit a rise intemperature.

After passing through a butterfly control valve 42, which controls theflow of air, which in turn controls the temperature of the prillsexiting the cooler, the air passes into the bottom chamber of air cooler36, which has a 6-foot diameter, is 30 feet high (straight side) andcontains 625 cubic feet of 2-inch raschig rings (a packing of 22 feet).The air is cooled by circulating 100 gpm of 25 percent urea solutiondownward through the packed bed air cooler, countercurrent to the airbeing cooled. The air passes upward through the packing 48, through ademister 49, and is recycled to the cooler 30 via line 37. The ureasolution enters the cooler at an inlet 43 at 60 F. and exits via outlet44 at 97 F. whence it is pumped via pump 45 to urea chiller 46, which isrefrigerated by means of an ammonia refrigeration unit (not shown). Theinjection system 47 provides optional urea coating agent which, whenused, discharges into air line 37 and enters the cooler 30 at the airinlet port 38.

FIG. 4 shows the air flow versus air temperature rise in a moving bedprill cooler such as cooler 30 shown in FIG. 3. This curve is based onthe heat necessary to be removed to cool the 400 T/D of urea prills atthe given conditions. In this case the net heat removed is 781,800 Btuper hour, and the design air flow rate is set for an 80 F. airtemperature rise.

FIG. 5 shows the terminal velocity in feet per second versus urea prilldiameter in microns. This curve will give an indication of the prillsize that will carry over in an air stream of a given velocity. FIG. 5is actually a combination of two graphs, one depicting Stokes Law andthe other an intermediate law. The curve shows both laws plotted on loglog scale. It has been found that Stokes Law is not applicable when theprills diameter is greater than about 200 microns. It has also beenfound that when the prill size is greater than 400 microns, the terminalvelocity of the prills in feet/second q l 0 415 g 0.5 where 1) equalsthe diameter g equals the specific gravity of the prills, in thisinstance 1.335. The area from 200 to 400 microns represents a transitionzone where neither law is in all cases, entirely accurate. However, thecurves have been joined and it has been found that there is not morethan about a 12.5 percent variation from numbers indicated by the curve.For a velocity of 2.74 ft./sec. in the prill cooler, all particlessmaller than 140 microns will be carried overhead.

As we have stated, anticaking characteristics of prills cooled by theprocess of this invention, although good, are further improved by theuse of conventional anticaking agents, particularly liquid agents. Theagents I herein discussed are particularly useful when applied atamounts in the range of 0.00250.5 percent based on the urea. Amongliquids, it has been found that n-octylamine is one of the best foroptional use in this invention.

n-Octylamine is commercially available as an amine mixture whichcomprises approximately 92 percent noctylarnine. At the temperature ofapplication, usually 68-70 F., its viscosity is suitable for applicationeither in a rotary mixer or by a pressure spray system. It is preferablyapplied to the prills at a level of 0.01-0.2 percent by weight.

Other particularly advantageous treating liquids are the monoethers ofpolyethylene-polypropyleneglycol, which are available commercially asthe so-called U- con fluids. The fluids are more and more hydrophobic asthe proportion of the polypropylene moiety is increased. Ucon fluidsprooved effective in extremely small proportions (0.0025 to 0.05 of 1percent).

Another class of fluids contributing anticaking characteristics to theprills is ethyleneglycol monobutyl ether, which is effective in rangesof from 0.01 to 0.05 of 1 percent by weight of the urea.

Upper limits for the weight of additives are not imposed for chemicalreasons, but are based on economics. The small increase in freedom fromcaking which is brought about by more than the amounts stated does notappear to warrant the cost of the extra treating agent involved.

EXAMPLES The static cooling unit described above in connection withFIGS. l-A, l-B, l-C, and 2 consisted of a inch X 20 inch X 38 inchplywood box with flanged connections at both top and bottom for airintroduction and removal. Two 50-mesh screens were mounted internally tosupport the prill charge. A forced air mover provided the necessarymovement of air through the urea prill bed. Connections were providedfor temperature and relative humidity instrumentation and for pressuredifferential measurements across the urea prill bed.

A hundred pounds of urea prill product exiting a commercial prill towerand having a temperature of about l50-l60 F. was charged into the staticcooling unit. The inlet air temperature was adjusted to range from 73 F.to 130 F. Time required to cool prills to set temperature (see Table Ibelow) was approximately 13 minutes.

After reaching the desired temperature, 50 pounds of urea prills werepackaged into standard 50-pound shipping bags. The test material wasplaced on a test platform in the warehouse and weighted with 1,000pounds of concrete blocks for 7 days. The weight was then removed, andthe bag carefully placed on 4-mesh screen mounted on a shaking device (aGump Shaker). One side and both ends are opened and the contents throughthe screen at this point is discarded.

The Hardness Index is the time in seconds required for the lumpedmaterial to be abraded through the 4- mesh screen by operating the GumpShaker at rpm. All material so abraded through the screen is collectedand weighed. The weight is expressed as percent Lump (on the startingweight of 50 pounds).

Thus, there is an inherent correlation in percent Lump and HardnessIndex, and it will be obvious that, in a qualitative way, the greaterthe caking, the greater the percent lumps, and the greater the HardnessIndex. All three characteristics are substantially lowered by use of thestatic cooler of this invention.

Fifty pounds of urea prills from a standard commercial rotary drumcooler were packaged into standard 50-pound shipping bags and subjectedto standard warehouse caking and hardness tests. (Examples 1 and 2 inTable I below.) The resulting difference in caking tendency and hardnessof cake in prills from the commercial cooler and prills from the staticcooler of this invention (Examples 3 and 4 is shown in Table I.

As already mentioned, further improvement in anticaking is obtained byadding certain liquids to the urea prills cooled in accordance with thisinvention. These liquids can be added to such prills in any conventionalliquid-solid mixing apparatus. A continuous r0- tary mixer is suitable.For small batches, even a concrete mixer can be used. Results of runsusing these liquids, are given below. The additives were used at 0.01percent level.

NOTE:

UCON LB is polyethylene-polypropylene glycol monoether, availablecommercially from Union Car bide Corp.

ARMEEN 8D is an amine mixture, 92 percent noctylamine, 3 percenthexylamine, and 5 percent decylamine, available commercially from ArmourChemical Co.

EGWE is ethylene glycol monobutyl ether.

As the terms are used herein, static cooling refers to a batch coolingoperation in which the prills do not move while they are being cooled;and semi-static cooling or continuous cooling refers to a coolingoperation in which cooling air is passed through a constant-depth bed ofprills, which bed is discharging prills below at substantially the samerate as it receives prills from above. Both types of cooling, as appliedin this invention operate in the same way, with minimal abrasion to theprill surface.

'In certain preferred embodiments of this invention the hot urea prillsare received into the cooling chamber from the prill tower at atemperature in the range of about 150-160 F. Cooling air is passedthrough the prill bed at velocities sufficient to cool the prills, or atleast the exiting prills, down to a temperature within the range ofabout 70-130 F., and more preferably down to a temperature in the rangeof about 70-8O F. This cooling is accomplished by using a stream of airhaving a temperature of 130 F. or below, e.g., down to 35 F. or evenlower, and more preferably a temperature below about 6080 F. In general,the lower the temperatures of the cooling air, the better,

but of course it should not chill the prills to the point where theywill condense moisture from the ambient air. The residence time of theprills within the cooler can be 10-60 minutes, but is preferably about-30 minutes.

What is claimed is:

1. In a process for preparing urea prills of reduced caking in a heatedprilling tower which provides hot solid prills and then cooling saidprills, that improvement which comprises collecting the hot solid prillsin a closed chamber cooler and directing a flow of air at a temperaturenot exceeding about 130 F. through the prills at an air velocity suchthat the prill temperature is reduced to below about 130 F. within aboutl0-60 minutes, and throughout the duration of cooling maintaining thecollected prills in a substantially quiescent state, whereby anunabraded glassy surface is main- I tained on each prill and subsequentcaking is reduced.

2. The process according to claim 1, in which the closed chamber cooleris a static cooler in which the prills do not move while they are beingcooled.

3. The process according to claim 1, in which the temperature of the airpassed into the cooler is in the range not exceeding about 6080 F. andthe temperature of the prills entering the cooler is about 150-160 F.and leaving the cooler does not exceed about -130bL F.

4. The process according to claim 1, in which the cooling chamber is acontinuous cooler in which the prills are constantly being received bysaid cooler and discharged by said cooler at the same rate, withoutsubstantial abrasion.

5. The process according to claim 4, in which the cooling air enters thecooler at about 70 F. and leaves it about F.

6. The process of claim 1, in which the cooled prills are treated with0.0025-0.5 percent of a liquid anticaking additive selected from thegroup consisting of polyethylene-polypropylene glycol monoether,n-octylamine, and ethylene glycol monobutyl ether to coat the prillswith said additive, to reduce caking further.

7. The process of claim 6, wherein the prills are treated with 0.01 to0.2 percent of n-octylamine.

8. The process of claim 6, wherein the prills are treated with 0.0025percent to 0.05 percent of the monoether of mixedpolyethylene-polypropyleneglycols.

9. The process of claim 6, wherein the prills are treated with 0.01 to0.5 percent of ethylene glycol monobutyl ether.

2. The process according to claim 1, in which the closed chamber cooleris a static cooler in which the Prills do not move while they are beingcooled.
 3. The process according to claim 1, in which the temperature ofthe air passed into the cooler is in the range not exceeding about60*-80* F. and the temperature of the prills entering the cooler isabout 150*-160* F. and leaving the cooler does not exceed about 70*-130*F.
 4. The process according to claim 1, in which the cooling chamber isa continuous cooler in which the prills are constantly being received bysaid cooler and discharged by said cooler at the same rate, withoutsubstantial abrasion.
 5. The process according to claim 4, in which thecooling air enters the cooler at about 70* F. and leaves it about 150*F.
 6. The process of claim 1, in which the cooled prills are treatedwith 0.0025-0.5 percent of a liquid anticaking additive selected fromthe group consisting of polyethylene-polypropylene glycol monoether,n-octylamine, and ethylene glycol monobutyl ether to coat the prillswith said additive, to reduce caking further.
 7. The process of claim 6,wherein the prills are treated with 0.01 to 0.2 percent of n-octylamine.8. The process of claim 6, wherein the prills are treated with 0.0025percent to 0.05 percent of the monoether of mixedpolyethylene-polypropylene-glycols.
 9. The process of claim 6, whereinthe prills are treated with 0.01 to 0.5 percent of ethylene glycolmonobutyl ether.